Low drop-out (LDO) voltage regulators are used for applications in which the difference between supply voltage and regulated voltage (drop-out) is relatively low--0.5 volts and less. Automotive electronics is a common application for such voltage regulators.
The specific problem to which the invention has applicability is the design of a LDO voltage regulator with (a) enhanced frequency stability (minimum AC ripple on the output), (b) the ability to operate at relatively high temperatures (around 175.degree. C.), and (c) protection from saturation and/or early-voltage mismatch errors associated with the use of bandgap voltage reference circuits.
LDO voltage regulators commonly use relatively large PNP transistors in the output stage because of the need to supply output current even under low drop-out conditions (i.e., conditions where the headroom required for an NPN output stage could not be provided). A disadvantage is that lateral PNP transistors used in typical N-epitaxial processes are characterized by relatively low transition frequencies compared to NPN transistors (which is why non-LDO voltage regulators typically use NPN transistors in the output stage). As a result, the phase margin for the voltage regulator is relatively small, introducing stability problems that can cause AC ripple on the output, or even operational instability.
Temperature stability is another problem for low drop-out voltage regulators operating with a wide range of output PNP collector current. As temperature rises significantly above about 125.degree. C., junction leakages cause the internal reference to drift upward such that, under conditions of low output collector current, the base of the transistor must be supplied with hole current to maintain regulation.
The common use of semiconductor bandgap voltage reference circuits to provide a voltage reference creates additional problems in designing an appropriate interface to the gain circuitry over a wide range of operating conditions. A typical approach is a single-sided interface. Due to early-voltage effects, the currents in the two legs of the reference circuit are susceptible to mismatch errors. Also, during start-up or an externally-forced deregulation, the transistors in one leg of the reference circuit can be forced into saturation, thereby preventing normal voltage reference operation.
Accordingly, a need exists for an LDO voltage regulator design that provides AC stabilization to minimize AC ripple on the output, temperature compensation to allow operation significantly above 125.degree. C., and for those voltage regulators using a bandgap voltage reference, a bandgap interface that reduces early voltage mismatch errors and prevents saturation.